Natural grass turf has been traditionally cultivated on playing surfaces for sporting events and athletic games. Natural grass turf is generally preferred over hard surfaces, for example, since it provides some resilience and cushioning for foot impacts and games where players frequently fall on the playing surface, such as football and soccer. The natural grass stabilizes the underlying soil to minimize problems with mud and dust, and provides an attractive appearance.
Maintenance of natural grass turf on athletic playing areas is expensive, natural grass does not grow well within shaded enclosed stadiums and continuous heavy traffic wears out certain spots in the turf surface making it extremely difficult to prevent accumulation of water and mud.
Synthetic turf therefore has been developed in order to reduce the expenses of maintaining athletic playing areas, and to increase the durability of the turf surface, especially where professional sports are involved.
Synthetic turf generally involves a carpet-like pile fabric with a flexible backing laid on a compacted substrate, such as crushed stone or other stabilized base material. The pile fabric has rows of upstanding synthetic ribbons representing glass blades extending upwardly from the top surface of the backing. Of particular interest to the present invention are the various formulations for granular resilient fill that is placed between the upstanding ribbons on the upper surface of the backing to simulate the presence of soil. Most prior art systems involve some use of sand or crushed slag particles, together with a resilient foam backing or crumb rubber particles to provide resilience.
For example, U.S. Pat. No. 3,995,079 to Haas, Jr. discloses a use of a turf pile fabric for covering a golf green. The infill is a selection from granulated coal slag, crushed flint or crushed granite. A foam resilient underpad provides some resilience, however, the angular particles of the infill are relatively abrasive. Where abrasion is a problem such as games of football, rugby, soccer, field hockey, baseball and other games where players may fall down or be knocked down on the playing surface, there is a need to provide resilient materials which are not abrasive on the granular infill. For example, U.S. Pat. No. 4,337,283 to Haas, Jr. discloses mixing of fine hard sand particles with 25% to 95% by volume resilient particles to provide an improved resilient and non-abrasive soil imitating infill. Such resilient material may include mixtures of granulated rubber particles, cork polymer beads, foam rubber particles, vermiculite, and the like.
A number of disadvantages result from the use of a uniformly mixed granular infill as in prior art systems where hard sand particles and resilient rubber particles are mixed in a uniform manner throughout the depth of the infill. Synthetic grass turf infill, for example, may comprise a mixture of 60% by weight of sand and 40% granulated rubber particles uniformly mixed and deposited between the upstanding synthetic grass ribbons to a depth of 1 to 3 inches. A high percentage of sand is preferred to minimize the cost of such systems, since rubber particles are relatively expensive compared to sand. The sand particles also provide an improved degree of drainage that is needed where the synthetic grass surface is not in an enclosed stadium for example. Rubber particles tend to impede the free flow of water, whereas the capillary action of the sand particles draws surface moisture downwardly due to the differences in surface tension characteristics between rubber and silica sand.
Prior art infill systems fail to recognize that the infill is a dynamic system continuously in movement under the influence of bouncing balls, vibration and impact from the feet and bodies of players in contact with the top surface of the infill. For example, a uniformly mixed infill with high proportion of sand will result in spraying of sand particles when a ball or player impacts with the top surface of the infill. Over time, areas of continuous impact will separate and sand will be visible. It is considered undesirable to have light colored sand visible in the synthetic grass surface and, especially when clouds of sand are visible on such impacts. In addition, exposed sand granules are abrasive to the skin when players fall or slide on the top surface.
Particularly in the case of relatively thin layers of infill, the infill layers of sand and rubber tend to sort themselves and compact into relatively firm surfaces. Therefore, when initially installed, the uniformly mixed infill will provide an adequate degree of resilience, however, over time the resilience decays to the point where the surface is firm and compacted. To avoid this problem, it is possible to install a thicker layer of infill, however, the resulting surface can be too resilient and may result in injury to players. Quite often the main complaints of professional athletes are that cleats on shoes do not release consistently from tightly woven or knitted synthetic sport turf surfaces, causing knee and ankle injuries and the synthetic surface is hard and abrasive, causing skin bums and abrasions. Granular infill addresses these disadvantages by providing a synthetic surface that better imitates a natural soil and turf.
A further disadvantage of uniformly mixed infills is that abrasive sand particles remain on the top surface of the synthetic turf and players on the surface who come in contact with the sand particles experience skin abrasion. Over time, due to the dynamics of vibration and impact, the smaller sand particles will tend to settle toward the bottom of the infill layer and larger more abrasive sand particles will rise to the top surface. As a result, over time the abrasive nature of the synthetic system is increased and may result in particular areas of the playing surface which experience heavy traffic being more abrasive than other areas.
It is an object of the present invention to provide an infill that will retain its properties throughout use. It is a further object of the invention to reduce the abrasive nature of synthetic turf infills. A further object of the invention is to stabilize the top surface of the infill to retain a resilient grass like surface that does not deteriorate in quality or compact over time through use.